This invention relates to screening devices for separating liquids and dispersed solids from liquid slurries, and in particular to an improved dewatering screen having a parabolically curved screening surface, and a method of fabricating the same.
Many screening devices of the so-called "Sidehill" type have been used for the gravity separation of solids from liquid slurries, or the thickening of such slurries, by introducing the slurry to a perforated, slotted or mesh screening surface disposed at some sloping angle to the horizontal. By introducing the slurries at the top of an inclined screening surface, liquids are induced by gravity to drain away from the solid materials contained in the slurry, with the mass becoming progressively thickened as it flows downward across the screening surface.
The classic sidehill screen with a wire mesh surface, although useful, tends to provide a slow rate of drainage, in addition to a tendency to "blind" over. To overcome these problems, some devices have employed a bar-type slotted screen surface. The screening bars are generally rectangular or triangular in cross-section, and are arranged on and fixed to a backing to provide slots of a controlled width which is substantially continuous throughout the screening surface in any given direction. These bars have been disposed both parallel and at angles ranging up to 90.degree. to the direction of flow of the incoming slurry to the inlet end of the screen.
Another approach has been screening devices of the constant or cylindrical curve type. These are generally premised on the assumption that the drainage characteristics of the slurry remain constant as the slurry passes downward on the curved surface, but the assumption has not proved to be true. Introduction of the slurry to the screening surface at a considerable velocity at the top thereof does provide rapid dewatering of the solids in the upper surfaces, but very little resistance to downward flow of the solids. As a result, a reduced dewatering rate occurs in the lower regions of the screen as the consistency of the slurry increases.
Thus none of the prior art devices or processes provide for the variability of the surface tension of the fluids in the slurry, the drainage rates of free water from the surface of the solid materials themselves, or the range of consistencies from incoming low consistency slurry to outgoing thickened slurry.
A need therefore exists for screening apparatus which takes into account the varying drainage rates, fluid surface tensions, and consistency ranges possessed by slurries as they are fed to and progress down dewatering screen surfaces.